Multiplexing circuit for liquid crystal display

Abstract

A multiplexed electrical control circuit is provided for use with optical display devices of the type wherein electrical intelligence is translated into optical images of letters or numbers or other patterns through the use of a liquid crystal shutter device fully described in a copending application Ser. No. 136,441, filed Apr. 22, 1971. The shutter device comprises a layer of liquid crystal material sandwiched between opposing parallel plates coated with transparent conducting films. The shutter device, or liquid crystal cell, is disposed between a pair of polarizers such that the application of electrical potential across a character, digit, or other image-defining segment and an opposing position-designating film will cause light to be transmitted or to be cut off, depending upon the orientation of the two polarizers. The multiplexing concept of the invention is based upon the discovery that the character, digit or other image-defining signal and the position-representing signal need to coincide only for a very brief period of time to switch a selected segment, in a given position, to an ON state. The switching is accomplished through a diode or other unidirectional conducting device so that the selected display element will hold its ON state until the voltage decays below the threshold as determined by the RC time constant. Each liquid crystal cell existing between a selected character segment and its selected opposing character position backing film then becomes an individually-actuable light shutter with a memory capacity due to its capacitive storage characteristic. As a result, it becomes possible to multiplex the control of many similar character or digit positions using the same character defining segment control signals.

We claim as our invention: 
     
       1. In an optical display system wherein electrical intelligence is converted into optical images through the use of a liquid crystal display, the combination of a layer of liquid crystal material sandwiched between a pair of transparent parallel plates, the inner surface of one of the plates in contact with the liquid crystal material having sets of mutually-insulated transparent conductive segments thereon which are arranged in essentially identical patterns, the inner surface of the other of said plates in contact with the liquid crystal material having patches of transparent conducting material thereon directly opposite cooperating sets of segments whereby establishment of an electrical potential between a segment and its cooperating patch on the opposite plate will change the optical appearance of that segment, there being n sets of segments and n patches on the opposite plate with x segments in each set, x conductors each connected to a corresponding one of the segments in each set, n conductors connected to the respective patches, means for applying pulses through the x conductors to the segments of all sets simultaneously in a sequence wherein the pulse sequence on the x conductors is adapted to change n times during a complete cycle, and means for applying during said cycle n pulses to said n conductors and the conducting patches, said n pulses being applied to the n conductors in succession and simultaneously with application of pulses to the x conductors whereby different configurations can be made to appear to the eye at successive segment locations. 
     
     
       2. The combination of claim 1 including unidirectional current devices in each of said x conductors. 
     
     
       3. The combination of claim 1 wherein said liquid crystal material is of the nematic type, means for producing a twisted nematic structure is said layer of liquid crystal material, and polarizers on opposite sides of the layer of liquid crystal material. 
     
     
       4. The combination of claim 3 wherein the pulses applied through the x conductors and said conducting patches each have two levels such that the potential difference between the voltage levels of the pulses when in their respective opposite levels will be sufficient to untwist said nematic structure. 
     
     
       5. The combination of claim 1 wherein the pulses applied to said conducting patches have a duration sufficient to charge the capacitance existing between the patch and its associated set of segments. 
     
     
       6. The combination of claim 5 wherein the discharge time constant of the capacitance existing between the patch and its associated set of segments is substantially longer than the time to complete a cycle such that each ON segment of a set of segments remains ON for a time while segments in another set are being actuated.